(1) Field of the Invention
The present invention relates to a method according to the preamble of claim 1, a data transfer system according to the preamble of claim 13, and a wireless data transfer device according to the preamble of claim 18.
(2) Description of Related art including information disclosed under 37 CFR 1.97 and 1.98
The term “wireless data transfer system” is generally used to mean any data transfer system, which enables a wireless data transfer connection between a wireless data transfer device (MS) and fixed parts of the system when the user of the wireless data transfer device is moving in the operating region of the system. A typical wireless data transfer system is the Public Land Mobile Network PLMN. Most of the wireless data transfer systems that exist at the time of filing this application belong to so-called second generation wireless data transfer device systems, an example of which is the widely known GSM system (Global System for Mobile telecommunications). The present invention is especially suitable for the packet-switched wireless data transfer device systems being developed now. An example of these wireless data transfer device systems used in this specification is the GPRS system (General Packet Radio Service), the standardization of which is under way now. It is clear that the invention can also be applied in other wireless data transfer device systems in which packet-switched data transfer is applied.
The General Packet Radio Service (GPRS) is a new service being developed for the GSM wireless data transfer device system. The operational environment of the GPRS system comprises one or more subnetwork service areas, which are combined as a GPRS backbone network. The subnetwork comprises several Support Nodes (SN), examples of which used in this specification are the Serving GPRS Support Nodes (SGSN), which are connected to the mobile network (typically via a connection unit to the base station) so that they can offer packet switched services to the wireless data transfer devices via the base stations (cells). The mobile network offers packet-switched information transfer between the support node and the wireless data transfer device. Different subnetworks, in turn, are connected via the GPRS Gateway Support Nodes (GGSN) to an external data network, such as a Public Switched Data Network (PSDN). The GPRS service thus enables packet-switched transfer of information between a wireless data transfer device and an external data network, whereby certain parts of the mobile network form an access network.
In order to use the GPRS services, the wireless data transfer device performs at first a GPRS attach, by which it notifies that it is ready for the transmission of packet data. The attach forms a logical link between the wireless data transfer device and the support node SGSN, and thus enables the transmission of short messages (SMS, Short Message Services) via the GPRS network, paging via a support node and notification of packet data to the wireless data transfer device. While the wireless data transfer device is attaching to the network, the support node performs the mobility management (MM) operation and user identification. In order to transmit and receive information, a Packet Data Protocol (PDP) is activated, whereby a packet data address to be used in a packet data connection is specified for the wireless data transfer device, and thus the address of the wireless data transfer device is known in the gateway GPRS support node. When the attach is performed, a data transfer connection is established with the wireless data transfer device, the support node and the gateway GPRS support node, and a protocol (such as (X.25 or IP), a connection address (e.g. X.121 address), Quality of Service and Network Service Access Point Identifier (NSAPI) are specified for the connection. The wireless data transfer device activates a packet data connection with an Activate PDP Context Request, in which the wireless data transfer device gives the Temporary Logical Link Identity (TLLI), the type of the packet data connection, the address, the required Quality of Service, the Network Service Access Point Identifier and possibly also the Access Point Name (APN).
The Quality of Service specifies, for instance, how Packet Data Units (PDU) are handled during the transfer in the GPRS network. The Qualities of Service defined for connection addresses, for example, are used to control the order of transmission, buffering (packet queues) and the rejection of packets in the support node and the gateway GPRS support node especially in situations where there are packets to be sent in two or more connections simultaneously. Different qualities of service specify different delays for the transfer of packets between different ends of the connection, different bit rates, and the number of packets rejected may be different in connections with different qualities of service. Four different Quality of Service classes have been formed in the GPRS system, and these classes specify the Quality of Service offered by the LLC layer to the connection.
Reliability determines whether acknowledgement is used (ARQ) or not (no ARQ) in the Logical Link Control (LLC) and Radio Link Control (RLC) layer in data transfer. In addition, reliability determines whether protected mode is used in non-acknowledged data transfer, and whether the GPRS backbone network uses the TCP or UDP protocol in the transfer of packets that belong to the connection.
The attached FIG. 1 shows the operation of a known LLC protocol layer 101 in the wireless data transfer device and in the GPRS support node. Block 102 represents the operations of the known RLC/MAC (Radio Link Control/Media Access Control) layer that are needed between the LLC layer 101 and the wireless data transfer device (not shown). Correspondingly, block 103 represents the operations of the known BSSGP (Base Station Subsystem GPRS Part) layer that are needed between the LLC layer 101 and the closest serving GPRS support node (not shown). The interface between the LLC layer 101 and the RLC/MAC layers is called the RR interface, and the interface between the LLC layer 101 and the BSSGP layers is called the BSSGP interface.
Above the LLC layer 101, there are the known GPRS mobility management operations 104, the SNDCP operations 105 and the short message service operations 106, which belong to layer 3 in the layered structure described here. Each of these blocks has one or more connection points to the LLC layer 101 for connecting to its different parts. The logical link control block 107 has a Logical Link-GPRS Mobility Management (LLGMM) control connection to block 104. The mobility management information is routed via the LLGMM connection between the blocks 104 and the first LLE (Logical Link Entity) block of the LLC layer. The second 109, third 110, fourth 111 and fifth 112 LLE block are connected to block 105 via corresponding connections. These blocks are also called QoS 1, QoS 2, QoS 3 and QoS 4 according to the Quality of Service of the packets handled by these blocks. The sixth LLE block 113 of the LLC layer is connected to the block 106 via the LLSMS (Logical Link-Short Message Service) connection. The Service Access Point Identifiers of the first 108, second 109, third 110, fourth 111, fifth 112 and sixth LLE block are 1, 3, 5, 9, 11 and 7, respectively. Each of these LLE blocks is linked in the LLC layer to the multiplexing block 114, which processes connections via the RR interface to block 102 and further to the wireless data transfer device, as well as connections via the BSSGP connection to block 103 and further towards the support node SGSN.
The connection between the multiplexing block 114 and block 102 of the lower level towards the wireless data transfer device is called the transmission pipe. All packet data flows between the upper parts of the LLC layer and the lower layers 102 go through the same multiplexing block 114 and transmission pipe. For the packet data transfer of the LLC layer 101 in the GPRS system, it is possible to create Temporary Block Flows (TBF) between the wireless data transfer device and the mobile network. Such a temporary block flow can be started either by the wireless data transfer device or the mobile network. These temporary block flows are temporary block flows of the RLC/MAC layer, in which information of the LLC layer is transferred. A temporary block flow may be intended for data transfer either from the mobile network to the wireless data transfer device, which is denoted shortly by DL TBF (Downlink TBF) in the signalling diagrams of FIGS. 2, 3a and 3b, or from the wireless data transfer device to the mobile network, in which case it is denoted by UL TBF (Uplink TBF).
FIG. 2 is a signalling diagram of prior art packet data transfer, in which temporary block flows are used. The block flow is preferably formed by means of a control channel, such as PCCCH or CCCH, by configuring a packet channel PDTCH. This is represented by block 201 in FIG. 2. When the temporary block flow has been formed, the transfer of packets is started (arrow 202). Each RLC packet sent by the mobile network to the wireless data transfer device contains a Final Block Indicator (FBI). The purpose of this final block indicator is to inform the wireless data transfer device when the mobile network no longer has information to be sent to the wireless data transfer device in the block flow, whereupon this temporary block flow can be stopped. In order to receive packets, the wireless data transfer device switches to the Packet Transfer Mode and starts listening to the packet data channel and receiving packets.
The mobile network sets information about this in the last packet to be transmitted (arrow 203), for example by setting the final bit of the packets in the packet header field to the value true (e.g. the logical mode 1). Then the wireless data transfer device knows that it was the last packet received in this block flow. This packet also contains the Relative Reserved Block Period (RRBP) field, in which the mobile network can inform the wireless data transfer device in which time slot the wireless data transfer device can send the acknowledge message. Having received this last packet, the wireless data transfer device transmits an acknowledgement message (204) to the mobile network in the given time slot and starts a timer (block 205), such as T3192 in the GPRS system, for time-out consideration. If the RLC Acknowledged Mode has been used in the block flow, the wireless data transfer device sends as acknowledgement the Packet Downlink Ack/Nack message, in which the Final Ack Indicator (FAI) is set to the value true, preferably the logical mode 1. The value of this final bit notifies the mobile network that retransmission of packets is not needed (any more), because all packets have been received. If the RLC Unacknowledged Mode has been used in the block flow, the wireless data transfer device sends the Packet Control Ack message as the acknowledgement message. The wireless data transfer device still continues listening to the packet data transfer channel PDTCH in case the wireless data transfer device would have to send the acknowledgement message again, until the time set in the timer T3192 has expired. After this, the wireless data transfer device switches to the idle state. A timer is also started in the mobile network, such as T3193 in the GPRS system, when the mobile network has received the acknowledgement message from the wireless data transfer device. After the time specified in the timer has expired, the mobile network releases the temporary block flow.
If the wireless data transfer device has packets to be sent in the idle mode, the wireless data transfer device cannot start sending these packets directly, but it must at first switch from the idle mode to the active mode (packet transmission mode). After this, the wireless data transfer device starts the procedure for forming a temporary block flow in the control channel, such as the above mentioned PCCCH or CCCH control channel (block 206). The transfer of packets from the wireless data transfer device to the mobile network can be started after the temporary block flow has been formed. The signalling performed during the formation is represented by arrows 207 and 208, and the configuration of the packet channel by block 209. The time needed for the request for resources and the formation of the temporary block flow may be as much as several seconds. In practice, an arrangement like the one described above delays the transfer of packets, because the wireless data transfer device must first wait for the end of the time-out consideration and switch to the idle mode before a new temporary block flow can be formed. In addition, establishing the connection causes extra loading of the control channel. Situations like the one described above arise especially in connection with the signalling processes, in which the wireless data transfer device must send a reply to a message sent by the mobile network substantially immediately.
If the mobile network has asked, in connection with the transmission of packets, the wireless data transfer device to send acknowledgement messages, the wireless data transfer device can inform the mobile network of the need to send packets in these acknowledgement messages. However, the mobile network does not always ask for acknowledgements, and so in a situation like this the wireless data transfer device does not have a chance to ask for resources for the transfer of packets before the acknowledgement after the reception of the last packet. Because at this stage the temporary block flow has ended and no transmission time slot has been reserved for the wireless data transfer device, the wireless data transfer device cannot send a request for resources. This means that the wireless data transfer device must switch to the idle mode and back before it can ask for resources for the transmission of packets.
Even when the acknowledged mode is used, there may be problems in sending the block flow request on time. The wireless data transfer device can set a Channel Request Description IE in the acknowledgement message, whereby the mobile network may try to allocate resources for establishing a temporary block flow from the wireless data transfer device to the mobile network. In this situation, the mobile network sends a resource allocation message (such as Packet Uplink Assignment) to the wireless data transfer device, after which the wireless data transfer device can start the transmission of packets. However, in all situations the wireless data transfer device does not have time to ask for the allocation of resources before it has to send the acknowledgement message. The reason for this may be, for instance, that packets of an application layer (e.g. information related to an Internet browser, such as information of a home page) are being transmitted in an RLC packet received by the wireless data transfer device in a block flow, in which case the packet must be moved from the RLC layer to the LLC layer. In the LLC layer, the LLC frame structure is dismantled and transferred to the TCP/IP layer via the SNDCP layer. From the TCP/IP layer, the information contained by the packet is transmitted to the application. After this, the application can form a reply message, for example, to be transmitted further via the mobile network. Reverse measures are then performed, i.e. the information of the application layer is transformed via the intermediate layers to information of the LLC layer and further to RLC packets. The time taken by this whole process may be so long that information about the need to transmit packets is not received in the RLC layer before the acknowledgement message of the RLC layer is sent to the mobile network.
In the GPRS system, the wireless data transfer device has a time of 13–26 TDMA frames (one frame is ca. 4.615 ms) to send an acknowledgement message. This time is influenced by the value of the RRBP field in the packet transmitted by the mobile network. This means that the upper layers of the protocol stack have approx. 60 to 120 ms of time to form the packet to be transmitted and move it to the RLC layer. In practical situations this does not often succeed, and thus the wireless data transfer device must first switch to the idle mode before a temporary packet flow from the wireless data transfer device to the mobile network can be formed.